Quoting Fly2HMO (Reply 2):So there's a gltich in my last post, but as I was saying... You MUST intercept the LOC before the G/S otherwise the A/P will not follow the G/S.

While it is a good idea to intercept the LOC before the G/S not doing so will not prevent the G/S capturing, at least not in every aircraft. The B747-400 for example has an autopilot option to disable G/S capture until after LOC capture. If not selected the G/S will capture before the LOC.

If you are way off the LOC the G/S gains will be not be ideal, so it may not capture and track too well.

The glass isn't half empty, or half full, it's twice as big as it needs to be.

I enable APP when on an intercept heading and cleared for the approach, with the proper frequency tuned and identified. Or, if it's a GPS approach, I could also just do it when cleared for the approach if I'm navigating toward something on the approach or if already established on a published portion.

Quoting Fly2HMO (Reply 4):Judging from the OP's other posts, I'm assuming this is a flightsim question in which case you do need to have the LOC acquired before the G/S.

I don't recall this being necessary in FS - you can capture GS before LOC, and the AP will start descending.

Intercepting the G/S before the LOC can get you violated since most places REQUIRE you to be ESTABLISHED on the final app course before leaving the final app alt. Obviously on a visual app this does not aply. All the jets I've flown require capturing the G/S from below as well.

If we're talking about the real world, many of us use 10nm as a rule of thumb. There's a guaranteed area of reliable coverage expressed in terms of angle and range, deviations being notified. Can't remember the dimensions. I go with the ten mile "rule" due to experience.

Tried a 17 mile G/S couple at CDG a few years ago. Observed the g/s pointer go steadily from above to below the centre, i.e. the aircraft had over-flown the G/S intercept. Said to the co-pilot "Look what its doing! Lets see what it does next" (it was 50 mile visibilty, no cloud). One half dot high, the aircraft (a classic 737) realised what was going on and ever so gently lowered the nose to a 2000+ fpm descent rate which would have caused a flap overspeed (we were flap 5 180 knots at ATC request) had I not disconnected. That was the aircraft's fault.

I find the 737 classics' A/P sloppier in pitch than any other airliner I have flown (ATRs, RJ100s [plus King Airs, Caravans, Do228s]), and indeed worse than some light aircraft I have flown.

Naples runway 24 is a rollercoaster if you engage g/s too far out, presumebly due to the terrain. I once did, just to see what happened, and we experienced level flight, gentle climbs and 2000 fpm descents with the associated huge thrust changes. I now use VNAV (with DME/Altitude checks) down to about 10 miles, which nails it.

Geneva runway 23 is disappointing also. Just y'day we had an unusually long 22 mile final. No terrain between us and the runway (its over the lake the whole way) and reasonable distance/height checks, so I engaged at about 15 miles. Not as bad as NAP but I won't be doing that again!

Quoting Fly2HMO (Reply 2): You MUST intercept the LOC before the G/S otherwise the A/P will not follow the G/S.

Not so on our 737 Classics and the RJ100s before that. Maybe a software tweak can prohibit it.

Quoting tdscanuck (Reply 11):So it's possible for G/S to engage without LOC engaging and still meet the requirement.

I guess so, IF an aircraft has a means of engaging glideslope without also coupling to the Loc. On the 737, to couple to the G/s you press the APP button. If there is a Loc detected, it will lock onto that also, even if the Loc wasn't engaged previously. Which types have seperate LOC and G/S buttons?

I believe I'm correct in saying that a G/S signal would never be radiated without its associated LOC??

The normal LOC coverage is 25 nm 10 degrees either side of the approach centerline and 17 nm out to 35 degrees either side. In some places, mainly in the US, this has been reduced to 18/10 nm in order to facilitate more facilities in a given area without frequency congestion. There are also extended range facilities, I know they exist with 75 nm coverage within ten degrees.

GP coverage is typically 10 nm and 8 degrees either side of the centerline. Outside the coverage volume there are no guarantees on the quality of the GP signal, meaning it is not legal to descend on the GP. The GP will typically droop as you diverge to either side, giving you a lower GP angle and reducing your obstacle clearance. That's worth keeping in mind. It is not rare to see a GP with reduced coverage. I'd strongly recommend to get that LOC needle close to centered before descending.

And no, you cannot have a GP without a LOC. That's also why you have an ident signal on the LOC but not on the GP.

Cheers,
/Fred

I thought I was doing good trying to avoid those airport hotels... and look at me now.

Quoting Fly2HMO (Reply 12):I can't think of any case that doesn't have that requirement to begin with.

I agree and I was saying that just to emphasis the point that if you're cleared for the ILS app and you're at the final app alt you need to be established on the loc before descending with the g/s.

Quoting tdscanuck (Reply 11):You can be on the final approach coarse without engaging LOC. So it's possible for G/S to engage without LOC engaging and still meet the requirement.

hey tdscanuk, I'm not sure I follow that. Are you saying you can be flying the loc but not tracking it? My comment was strictly referring to already being at the final app alt. Last night we were 1000' above F.A.A. and we were clearded for the visual (the ILS would be the same) though not on the loc we descended to the final app alt but no further until eswtablished.

Quoting CosmicCruiser (Reply 15):Quoting tdscanuck (Reply 11):
You can be on the final approach coarse without engaging LOC. So it's possible for G/S to engage without LOC engaging and still meet the requirement.

hey tdscanuk, I'm not sure I follow that. Are you saying you can be flying the loc but not tracking it?

"LOC" (in capitals and in the context of "engagement") is an autoflight mode. You can be flying on the localizer course ("on the loc") without the LOC mode engaging on the autoflight system (either to drive the flight director or the autopilot). It's like flying an ILS on an aircraft that doesn't have an autopilot or flight director. You're flying the proscribed course (and flying the glideslope, or not) but haven't actually engaged the relevant flight control modes.

I'm not aware that this happens as a part of regular operation, or even ever, but it's physically possible with some aircraft.

Quoting Fly2HMO (Reply 2): You MUST intercept the LOC before the G/S otherwise the A/P will not follow the G/S.

That is not true in all aircraft, most notably Boeing aircraft. Different aircraft handle this in different ways. There are techniques and procedures used to prevent this from happening. On Boeing aircraft I always arm LOC until the course is alive to prevent the capture of the GS first, especially when being very close in and at or close to the GS Intercept Altitude. Once the LOC is active I arm the approach, otherwise you get into the scenario CosmicCruiser was discussing where you can descend on the GS without lateral course guidance, which is a very bad move.

Quoting jetlagged (Reply 3):While it is a good idea to intercept the LOC before the G/S not doing so will not prevent the G/S capturing, at least not in every aircraft.

Quoting Fly2HMO (Reply 4):Judging from the OP's other posts, I'm assuming this is a flightsim question in which case you do need to have the LOC acquired before the G/S.

I am talking about the real aircraft; I have never used MSFS. If all aircraft behave this way in FS, then FS is incorrect.

Quoting PGNCS (Reply 18):That is not true in all aircraft, most notably Boeing aircraft.

It's an option on Boeing's. The default is to allow G/S to engage without LOC engaged (they'll both arm at the same time if you push the APP button but they can engage independantly), but you can select the option to inhibit G/S engage until LOC engages. Under the default configuration, to implement the same function, you just hit the LOC button instead of APP...that arms only LOC, while the pitch mode remains in whatever you had before. Then you're free to engage G/S whenever you want, later, by hitting APP.

Quoting tdscanuck (Reply 19):Quoting PGNCS (Reply 18):
That is not true in all aircraft, most notably Boeing aircraft.

It's an option on Boeing's. The default is to allow G/S to engage without LOC engaged (they'll both arm at the same time if you push the APP button but they can engage independantly), but you can select the option to inhibit G/S engage until LOC engages. Under the default configuration, to implement the same function, you just hit the LOC button instead of APP...that arms only LOC, while the pitch mode remains in whatever you had before. Then you're free to engage G/S whenever you want, later, by hitting APP.

Tom.

Thanks Tom. I can only speak as a pilot; the (multiple) operators I have worked for apparently selected the default option. I described in my earlier reply 18 the exact technique you have explained when in Boeing aircraft. To prevent the GS from capturing first, I select LOC and once the LOC is captured, arm the approach. The operational key is to note when the GS comes alive that the approach is armed, and if not, why not? It's all about the FMA.

Yeah, for instance if the approach fixes are loaded in the FMS and you're flying with FMS as your nav source. If you don't switch over to NAV, you'll fly along the localizer, but your track will be based off of where the FMS thinks the line connecting the fixes is, and not where the localizer is (though the two should be quite close).

Yes I'm aware of this we do it from time to time especially if it's a hard turn to final. FMS will round out the turn much better than LOC armed will. I just didn't follow his statement since I was talking about not descending on the g/s prior to loc established where I didn't really mean LOC armed. I could be hand flying using raw data but still need to be within a dot of centerline. Just my misunderstanding.